agp/intel: Respect the GTT size on Sandybridge for scratch page setup.

This is similar to 14bc490bbdf1b194ad1f5f3d2a0a27edfdf78986 which
respected it for how much of the GTT we would actually use.  Now we
won't clear beyond allocated memory when filling the GTT with scratch
page addresses.

Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/char/agp/intel-agp.c

@@ -175,6 +175,10 @@ extern int agp_memory_reserved;
 #define SNB_GMCH_GMS_STOLEN_448M	(0xe << 3)
 #define SNB_GMCH_GMS_STOLEN_480M	(0xf << 3)
 #define SNB_GMCH_GMS_STOLEN_512M	(0x10 << 3)
+#define SNB_GTT_SIZE_0M			(0 << 8)
+#define SNB_GTT_SIZE_1M			(1 << 8)
+#define SNB_GTT_SIZE_2M			(2 << 8)
+#define SNB_GTT_SIZE_MASK		(3 << 8)
 
 static const struct aper_size_info_fixed intel_i810_sizes[] =
 {
@@ -1438,6 +1442,8 @@ static unsigned long intel_i965_mask_memory(struct agp_bridge_data *bridge,
 
 static void intel_i965_get_gtt_range(int *gtt_offset, int *gtt_size)
 {
+	u16 snb_gmch_ctl;
+
 	switch (agp_bridge->dev->device) {
 	case PCI_DEVICE_ID_INTEL_GM45_HB:
 	case PCI_DEVICE_ID_INTEL_EAGLELAKE_HB:
@@ -1449,9 +1455,26 @@ static void intel_i965_get_gtt_range(int *gtt_offset, int *gtt_size)
 	case PCI_DEVICE_ID_INTEL_IRONLAKE_M_HB:
 	case PCI_DEVICE_ID_INTEL_IRONLAKE_MA_HB:
 	case PCI_DEVICE_ID_INTEL_IRONLAKE_MC2_HB:
+		*gtt_offset = *gtt_size = MB(2);
+		break;
 	case PCI_DEVICE_ID_INTEL_SANDYBRIDGE_HB:
 	case PCI_DEVICE_ID_INTEL_SANDYBRIDGE_M_HB:
-		*gtt_offset = *gtt_size = MB(2);
+		*gtt_offset = MB(2);
+
+		pci_read_config_word(intel_private.pcidev, SNB_GMCH_CTRL, &snb_gmch_ctl);
+		switch (snb_gmch_ctl & SNB_GTT_SIZE_MASK) {
+		default:
+		case SNB_GTT_SIZE_0M:
+			printk(KERN_ERR "Bad GTT size mask: 0x%04x.\n", snb_gmch_ctl);
+			*gtt_size = MB(0);
+			break;
+		case SNB_GTT_SIZE_1M:
+			*gtt_size = MB(1);
+			break;
+		case SNB_GTT_SIZE_2M:
+			*gtt_size = MB(2);
+			break;
+		}
 		break;
 	default:
 		*gtt_offset = *gtt_size = KB(512);